Method and apparatus providing a multi-function terminal for a power supply controller

ABSTRACT

A power supply controller having a multi-function terminal. In one embodiment, a power supply controller for switched mode power supply includes a drain terminal, a source terminal, a control terminal and a multi-function terminal. The multi-function terminal may be configured in a plurality of ways providing any one or some of a plurality of functions including on/off control, external current limit adjustments, under-voltage detection, over-voltage detection and maximum duty cycle adjustment. The operation of the multi-function terminal varies depending on whether a positive or negative current flows through the multi-function terminal. A short-circuit to ground from the multi-function terminal enables the power supply controller. A short-circuit to a supply voltage from the multi-function terminal disables the power supply controller. The current limit of an internal power switch of the power supply controller may be adjusted by externally setting a negative current from the multi-function terminal. The multi-function terminal may also be coupled to the input DC line voltage of the power supply through a resistance to detect an under-voltage condition, an over-voltage condition and/or adjust the maximum duty cycle of power supply controller. Synchronization of the oscillator of the power supply controller may also be realized by switching the multi-function terminal to power or ground at the desired times.

This is a Divisional of U.S. application Ser. No. 09/405,209, filed Sep.24, 1999, now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power supplies and, morespecifically, the present invention relates to a switched mode powersupply controller.

2. Background Information

Electronic devices use power to operate. Switched mode power suppliesare commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 Hz or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highvoltage direct current (DC) with a diode rectifier and capacitor. Thehigh voltage DC is then converted to high frequency (e.g. 30 to 300 kHz)AC, using a switched mode power supply control circuit. This highfrequency, high voltage AC is applied to a transformer to transform thevoltage, usually to a lower voltage, and to provide safety isolation.The output of the transformer is rectified to provide a regulated DCoutput, which may be used to power an electronic device. The switchedmode power supply control circuit provides usually output regulation bysensing the output controlling it in a closed loop.

A switched mode power supply may include an integrated circuit powersupply controller coupled in series with a primary winding of thetransformer. Energy is transferred to a secondary winding from theprimary winding in a manner controlled by the power supply controller toprovide the clean and steady source of power at the DC output. Thetransformer of a switched mode power supply may also include anotherwinding called a bias or feedback winding. The bias winding provides theoperating power for the power supply controller and in some cases italso provides a feedback or control signal to the power supplycontroller. In some switched mode power supplies, the feedback orcontrol signal can come through an opto-coupler from a sense circuitcoupled to the DC output. The feedback or control signal may be used tomodulate a duty cycle of a switching waveform generated by the powersupply controller or may be used to disable some of the cycles of theswitching waveform generated by the power supply controller to controlthe DC output voltage.

A power supply designer may desire to configure the power supplycontroller of a switched mode power supply in a variety of differentways, depending on for example the particular application and/oroperating conditions. For instance, there may be one application inwhich the power supply designer would like the power supply controllerto have one particular functionality and there may be anotherapplication in which the power supply designer would like the powersupply controller to have another particular functionality. It would beconvenient for power supply designer to be able to use the sameintegrated power supply controller for these different functions.

In order to provide the specific functions to the power supplycontroller, additional pins or electrical terminals are added for eachfunction to the integrated circuit power supply controllers.Consequently, each additional function generally translates into anadditional pin on the power supply controller chip, which translatesinto increased costs and additional external components. Anotherconsequence of providing additional functionality to power supplycontrollers is that there is sometimes a substantial increase in powerconsumption by providing the additional functionality.

SUMMARY OF THE INVENTION

Power supply controller methods and apparatuses are disclosed. In oneembodiment, a power supply controller circuit is described including acurrent input circuit coupled to receive a current. In one embodiment,the current input circuit is to generate an enable/disable signal inresponse to the current. The power supply controller is to activate anddeactivate the power supply in response to the enable/disable signal. Inanother embodiment, a current limit of a power switch of the powersupply controller is adjusted in response to the current. In yet anotherembodiment, a maximum duty cycle of the power switch of the power supplyis adjusted in response to the current. Additional features and benefitsof the present invention will become apparent from the detaileddescription, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic illustrating one embodiment of a power supplyincluding a power supply controller having a multi-function terminal inaccordance with the teachings of the present invention.

FIG. 2A is a schematic illustrating one embodiment of a power supplycontroller having a multi-function terminal configured to limit thecurrent of the power switch in the power supply controller to a desiredvalue in accordance with the teachings of the present invention.

FIG. 2B is a schematic illustrating one embodiment of a power supplyhaving a multi-function terminal configured to provide a switchableon/off control to the power supply in accordance with the teachings ofthe present invention.

FIG. 2C is a schematic illustrating one embodiment of the power supplyhaving a multi-function terminal configured to limit the current of thepower switch in the power supply controller to a desired value andprovide a switchable on/off control to the power supply controller inaccordance with the teachings of the present invention.

FIG. 2D is a schematic illustrating one embodiment of a power supplyhaving a multi-function terminal configured to provide lineunder-voltage detection, line over-voltage detection and maximum dutycycle reduction of the power supply in accordance with the teachings ofthe present invention.

FIG. 2E is a schematic illustrating one embodiment of a power supplyhaving a multi-function terminal configured to provide lineunder-voltage detection, line over-voltage detection, maximum duty cyclereduction and a switchable on/off control to the power supply inaccordance with the teachings of the present invention.

FIG. 2F is a schematic illustrating one embodiment of current modecontrol of a power supply controller having a multi-function terminalconfigured to regulate the current limit of the power switch in responseto the power supply output in accordance with the teachings of thepresent invention.

FIG. 3 is a block diagram illustrating one embodiment of a power supplycontroller including a multi-function terminal in accordance withteachings of the present invention.

FIG. 4 is a schematic illustrating one embodiment of a power supplycontroller including a multi-function terminal in accordance with theteachings of the present invention.

FIG. 5 is a diagram illustrating one embodiment of currents, voltagesand duty cycles in relation to current through a multi-function terminalof a power supply controller in accordance with teachings of the presentinvention.

FIG. 6A is a diagram illustrating one embodiment of timing diagrams ofswitching waveforms of a power supply controller including amulti-function terminal in accordance with teachings of the presentinvention.

FIG. 6B is a diagram illustrating another embodiment of timing diagramsof switching waveforms of the power supply controller including amulti-function terminal in accordance with teachings of the presentinvention.

FIG. 7 is a schematic illustrating another embodiment of a power supplycontroller including a multi-function terminal in accordance with theteachings of the present invention.

FIG. 8 is a diagram illustrating another embodiment of timing diagramsof switching waveforms of the power supply controller including amulti-function terminal in accordance with teachings of the presentinvention.

DETAILED DESCRIPTION

A method and an apparatus providing a multi-function terminal in a powersupply controller is disclosed. In the following description, numerousspecifically details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone having ordinary skill in the art that the specific detail need notbe employed to practice the present invention. In other instances,well-known materials or methods have not been described in detail inorder to avoid obscuring the present invention.

In one embodiment of the present invention, a power supply controller isprovided with the functionality of being able to remotely turn on andoff the power supply. In another embodiment, the power supply controlleris provided with the functionality of being able to externally set thecurrent limit of a power switch in the power supply controller, whichmakes it easier to prevent saturation of the transformer reducingtransformer size and cost. Externally settable current limit also allowsthe maximum power output to be kept constant over a wide input rangereducing the cost of components that would otherwise have to handle theexcessive power at high input voltages. In yet another embodiment, thepower supply controller is provided with the functionality of being ableto detect an under-voltage condition in the input line voltage of thepower supply so that the power supply can be shutdown gracefully withoutany glitches on the output. In still another embodiment, the powersupply controller is provided with the functionality of being able todetect an over-voltage condition in the input line voltage of the powersupply so that the power supply can be shut down under this abnormalcondition. This allows the power supply to handle much higher surgevoltages due to the absence of reflected voltage and switchingtransients on the power switch in the power supply controller. Inanother embodiment, the power supply controller is provided with thefunctionality of being able to limit the maximum duty cycle of aswitching waveform generated by a power supply controller to control theDC output of the power supply. In so doing, saturation of thetransformer during power up is reduced and the excess power capabilityat high input voltages is safely limited. Increased duty cycle at low DCinput voltages also allows for smaller input filter capacitance. Thus,this feature results in cost savings on many components in the powersupply including the transformer. In yet another embodiment, some or allof the above functions are provided with a single multi-functionterminal in the power supply controller. That is, in one embodiment, aplurality of additional functions are provided to power supplycontroller without the consequence of adding a corresponding pluralityof additional terminals or pins to the integrated circuit package of thepower supply controller. In one embodiment, one or some of the abovefunctions are available when positive current flows into themulti-function terminal. In another embodiment, one or some of the abovefunctions are available when negative current flows out from themulti-function terminal. In one embodiment, the voltage at themulti-function terminal is fixed at a particular value depending onwhether positive current flows into the multi-function terminal orwhether negative current flows out from the multi-function terminal.

The multi-function features listed above not only save cost of manycomponents and improve power supply performance but also, they save manycomponents that would otherwise be required if these features wereimplemented externally.

FIG. 1 is a block diagram illustrating one embodiment of a power supply101 including a power supply controller 139 having a multi-functionterminal 149 in accordance with the teachings of the present invention.As illustrated, power supply 101 includes an AC mains input 103, whichis configured to receive an AC voltage input. A diode rectifier 105 iscoupled to AC mains input to rectify the AC voltage. Capacitor 107 iscoupled to diode rectifier 105 to convert the rectified AC into a steadyDC line voltage 109, which is coupled to a primary winding 111 of atransformer. Zener diode 117 and diode 119 are coupled across primarywinding 111 to provide clamp circuitry.

As illustrated in FIG. 1, primary winding 111 is coupled to a drainterminal 141 of power supply controller 139. Power supply controller 139includes a power switch 147 coupled between the drain terminal 141 and asource terminal 143, which is coupled to ground. When power switch 147is turned on, current flows through primary winding 111 of thetransformer. When current flows through primary winding 111, energy isstored in the transformer. When power switch 147 is turned off, currentdoes not flow through primary winding 111 and the energy stored in thetransformer is transferred to secondary winding 113 and bias winding115.

A DC output voltage is produced at DC output 125 through diode 121 andcapacitor 123. Zener diode 127, resistor 129 and opto-coupler 131 formfeedback circuitry or regulator circuitry to produce a feedback signalreceived at a control terminal 145 of the power supply controller 139.The feedback or control signal is used to regulate or control thevoltage at DC output 125. As the voltage across DC output 125 risesabove a threshold voltage determined by Zener diode 127, resistor 129and opto-coupler 131, additional feedback current flows into controlterminal 145. In one embodiment, control terminal 145 also provides asupply voltage for circuitry of power supply controller 139 through biaswinding 115, diode 133, capacitor 135 and capacitor 137.

As shown in FIG. 1, power supply controller 139 includes amulti-function terminal 149, which in one embodiment enables powersupply controller 139 to provide one or a plurality of differentfunctions, depending on how multi-function terminal 149 is configured.Some examples of how multi-function terminal 149 may be configured areshown in FIGS. 2A through 2F.

For instance, FIG. 2A is a diagram illustrating one embodiment of apower supply controller 139 including a resistor 201 coupled between themulti-function terminal 149 and the source terminal 143. In oneembodiment, the source terminal 143 is coupled to ground. In oneembodiment, the voltage at multi-function terminal 149 is fixed whennegative current flows from multi-function terminal 149. In oneembodiment, the negative current that flows through resistor 201 is usedto set externally the current limit of power switch 147. Thus, the powersupply designer can choose a particular resistance for resistor 201 toset externally the current limit of power switch 147. In one embodiment,resistor 201 may be a variable resistor, a binary weighted chain ofresistors or the like. In such embodiment, the current limit of powerswitch 147 may be adjusted externally by varying the resistance ofresistor 201. In one embodiment, the current limit of power switch 147is directly proportional to the negative current flowing throughresistor 201.

FIG. 2B is a diagram illustrating another embodiment of a power supplycontroller 139 including a switch 203 coupled between multi-functionterminal 149 and source terminal 143. In one embodiment, source terminal143 is coupled to ground. In one embodiment, power supply controller 139switches power switch 147 when multi-function terminal 149 is coupled toground through switch 203. In one embodiment, power supply controller139 does not switch power switch 147 when multi-function terminal 149 isdisconnected from ground through switch 203. In particular, when anadequate amount of negative current flows from multi-function terminal149, power supply 101 is enabled. When substantially no current flowsfrom multi-function terminal 149, power supply 101 is disabled. In oneembodiment, the amount of current that flows from multi-functionterminal 149 to ground through switch 203 is limited. Thus, in oneembodiment, even if multi-function terminal 149 is short-circuited toground through switch 203, the amount of current flowing frommulti-function terminal 149 to ground is limited to a safe amount.

FIG. 2C is a diagram illustrating yet another embodiment of a powersupply controller 139 including resistor 201 and switch 203 coupled inseries between multi-function terminal 149 and source terminal 143,which in one embodiment is ground. The configuration illustrated in FIG.2C combines the functions illustrated and described in connection withFIGS. 2A and 2B above. That is, the configuration illustrated in FIG. 2Cillustrates a power supply controller 139 having external adjustment ofthe current limit of power switch 147, through the selection of theresistance for resistor 201, and on/off functionality through switch203. When switch 203 is on, power supply controller 139 will switchpower switch 147 with a current limit set by resistor 201. When switch203 is off, power supply controller 139 will not switch power switch 147and power supply 101 will be disabled.

FIG. 2D is a diagram illustrating still another embodiment of a powersupply controller 139 including a resistor 205 coupled between the linevoltage 109 and multi-function terminal 149. Referring briefly back toFIG. 1 above, DC line voltage 109 is generated at capacitor 107 and isinput to the primary winding 111 of the transformer of power supply 101.Referring back the FIG. 2D, in one embodiment, multi-function terminal149 is substantially fixed at a particular voltage when positive currentflows into multi-function terminal 149. Therefore, the amount ofpositive current flowing through resistor 205 into multi-functionterminal 149 is representative of line voltage 109, which is input tothe primary winding 111. Since the positive current flowing throughresistor 205 into multi-function terminal 149 represents the linevoltage 109, power supply controller 139 can use this positive currentto sense an under-voltage condition in line voltage 109 in oneembodiment. An under-voltage condition exists when the line voltage 109is below a particular under-voltage threshold value. In one embodiment,if a line under-voltage condition is detected, power switch 147 is notswitched by power supply controller 139 until the under-voltagecondition is removed.

In one embodiment, power supply controller 139 can use the positivecurrent flowing through resistor 205 into multi-function terminal 149 todetect an over-voltage condition in line voltage 109. An over-voltagecondition when line voltage 109 rises above a particular over-voltagethreshold value. In one embodiment, if a line over-voltage condition isdetected, power switch 147 is not switched by power supply controller139 until the over-voltage condition is removed.

In one embodiment, power supply controller 139 can also use the positivecurrent flowing through resistor 205 and multi-function terminal 149 todetect for increases or decreases in line voltage 109. As line voltage109 increases, for a given fixed maximum duty cycle, the maximum poweravailable to secondary winding 113 in power supply 101 of FIG. 1 usuallyincreases. As line voltage 109 decreases, less power is available tosecondary winding 113 in power supply 101. In most cases, the excesspower available at the DC output 125 is undesirable under overloadconditions due to high currents that need to be handled by components.In some instances, it is also desirable to increase the maximum poweravailable to DC output 125 at low input DC voltages to save on cost ofthe input filter capacitor 107. Higher duty cycle at low DC inputvoltage allows lower input voltage operation for a given output power.This allows larger ripple voltage on capacitor 107, which translates toa lower value capacitor. Therefore, in one embodiment, power supplycontroller 139 adjusts the maximum duty cycle of a switching waveformused to control or regulate power switch 147 in response to increases ordecreases in line voltage 109. In one embodiment, the maximum duty cycleof the switching waveform used to control power switch 147 is inverselyproportional to the line voltage 109. As mentioned earlier, reducing theduty cycle with increasing input DC voltage has many advantages. Forinstance, it reduces the value and hence the cost of capacitor 107. Inaddition, it limits excess power at high line voltages reducing the costof the clamp circuit (117, 119), the transformer and the outputrectifier 121 due to reduced maximum power ratings on these components.

It is appreciated that since only a single resistor 201 to ground, or asingle resistor 205 to line voltage 109, is utilized for implementingsome of the functions of power supply controller 139, a power savings isrealized. For instance, if a resistor divider were to be coupled betweenpower and ground, and a voltage output of the resistor divider coupledto a terminal of power supply controller 139 were to be used, currentwould continuously flow through both the resistor divider and into asensor terminal of the power supply controller. This would result inincreased power consumption. However in one embodiment of the powersupply controller 139, only the single resistor 201 to ground or singleresistor 205 to line voltage 109 is utilized, thereby eliminating theneed for a current to flow through both the resistor divider and intopower supply controller 139.

FIG. 2E is a diagram illustrating yet another embodiment of a powersupply controller 139 including resistor 205, as described above,coupled between the line voltage 109 and multi-function terminal 149.FIG. 2E also includes a switch 207 coupled between control terminal 145and multi-function terminal 149. In one embodiment, resistor 205provides the same functionality as discussed above in connection withFIG. 2D. Therefore, when switch 207 is switched off, the configurationillustrated in FIG. 2E is identical to the configuration described abovein connection with FIG. 2D.

In one embodiment, control terminal 145 provides a supply voltage forpower supply controller 139 in addition to providing a feedback orcontrol signal to power supply controller 139 from DC output 125. As aresult, in one embodiment, switch 207 provides in effect a switchablelow resistance connection between a supply voltage (control terminal145) and multi-function terminal 149. In one embodiment, the maximumpositive current that can flow into multi-function terminal 149 islimited. Therefore, in one embodiment, even when switch 207 provides, ineffect, a short-circuit connection from a supply voltage, the positivecurrent that flows into multi-function terminal 149 is limited to a safeamount. However, in one embodiment, the positive current that does flowthrough switch 207, when activated, into multi-function terminal 149triggers an over-voltage condition. As discussed above, power supply 139discontinues switching power switch 147 during an over-voltage conditionuntil the condition is removed. Therefore, switch 207 provides on/offfunctionality for power supply controller 139. When switch 207 is theactivated, the low resistance path to control terminal 145 is removedand the positive current flowing into multi-function terminal 149 islimited to the current that flows from line voltage 109 through resistor205. Assuming that neither an under-voltage condition nor anover-voltage condition exists, power supply controller 139 will resumeswitching power switch 147, thereby re-enabling power supply 101.

FIG. 2F is a diagram illustrating another embodiment of a power supplycontroller 139 using current mode control to regulate the current limitof the power supply. As shown, resistor 201 is coupled between themulti-function terminal 149 and the source terminal 143 and thetransistor 209 of an opto-coupler coupled between multi-functionterminal 149 and a bias supply, such as for example control terminal145. Similar to FIG. 2A, the negative current that flows out frommulti-function terminal 149 is used to set externally the current limitof power switch 147. In the embodiment illustrated in Figure in FIG. 2F,the current limit adjustment function can be used for controlling thepower supply output by feeding a feedback signal from the output of thepower supply into multi-function terminal 149. In the embodimentdepicted in FIG. 2F, the current limit is adjusted in a closed loop toregulate the output of the power supply (known as current mode control)by adding the opto-coupler output between multi-function terminal 149and the bias supply.

In one embodiment, the power supply controller configurations describedin connection with FIGS. 2A through 2F all utilize the samemulti-function terminal 149. Stated differently, in one embodiment, thesame power supply controller 139 may be utilized in all of theconfigurations described. Thus, the presently described power controller139 provides a power supply designer with added flexibility. As aresult, a power supply designer may implement more than one of the abovefunctions at the same time using the presently described power supplycontroller 139. In addition, the same functionality may be implementedin more than one way. For example, power supply 101 can be remotelyturned on and off using either power or ground. In particular, the powersupply 101 can be turned on and off by switching to and from the controlterminal (supply terminal for the power supply controller) using theover-voltage detection feature, or by switching to and from ground usingthe on/off circuitry.

FIGS. 2A though 2F provide just a few examples of use of themulti-function terminal. A person skilled in the art will find manyother configurations for use of the multi-function pin. The uses for themulti-function terminal, are therefore, not limited to the few examplesshown.

It is worthwhile to note that different functions of the presentlydescribed power supply controller 139 may be utilized at different timesduring different modes of operation of power supply controller 139. Forinstance, some features may be implemented during startup operation,other functions may be implemented during normal operation, otherfunctions may be implemented during fault conditions, while still otherfunctions may be implemented during standby operation. Indeed, it isappreciated that a power supply designer may implement other circuitconfigurations to use with a power supply controller 139 in accordancewith teachings of the present invention. The configurations illustratedin FIGS. 2A through 2F are provided simply for explanation purposes.

FIG. 3 is a block diagram illustrating one embodiment of a power supplycontroller 139 in accordance with teachings of the present invention. Asshown in the embodiment illustrated, power supply controller 139includes a current input circuit 302, which in one embodiment serves asmulti-function circuitry. In one embodiment, current input circuit 302includes a negative current input circuit 304 and a positive currentinput circuit 306. In one embodiment, negative current input circuit 304includes negative current sensor 301, on/off circuitry 309 and externalcurrent limit adjuster 313. In one embodiment, positive current inputcircuit 306 includes positive current sensor 305, under-voltagecomparator 317, over-voltage comparator 321 and maximum duty cycleadjuster 325.

As shown in FIG. 3, negative current sensor 301 and positive currentsensor 305 are coupled to multi-function terminal 149. In oneembodiment, negative current sensor 301 generates a negative currentsense signal 303 and positive current sensor generates a positivecurrent sense signal 307. For purposes of this description, a negativecurrent may be interpreted as current that flows out of multi-functionterminal 149. Positive current may be interpreted as current that flowsinto multi-function terminal 149. In one embodiment, on/off circuitry309 is coupled to receive negative current sense signal 303. Externalcurrent limit adjuster 313 is coupled to receive negative current sensesignal 303.

In one embodiment, under-voltage comparator 317 is coupled to receivepositive current sense signal 307. Over-voltage comparator 321 iscoupled to receive positive current sense signal 307. As discussedearlier, both under-voltage and over-voltage comparators also functionas on/off circuits. Maximum duty cycle adjuster 325 is also coupled toreceive positive current sense signal 307.

In one embodiment, on/off circuitry 309 generates an on/off signal 311,under-voltage comparator 317 generates an under-voltage signal 319 anover-voltage comparator 321 generates an over-voltage signal 323. Asshown in the embodiment illustrated in FIG. 3, enable/disable logic 329is coupled to receive the on/off signal 311, the under-voltage signal319 and the over-voltage signal 323. The under-voltage and over-voltagesignals can also be used for on/off functions as noted earlier.

In one embodiment, enable/disable logic 329 generates an enable/disablesignal 331, which is coupled to be received by control circuit 333. Thecontrol circuit 333 is also coupled to receive a control signal fromcontrol terminal 145. In addition, control circuit 333 is also coupledto receive a drain signal from drain terminal 141, a maximum duty cycleadjustment signal 327 from maximum duty cycle adjuster 325 and anexternal current limit adjustment signal 315 from external current limitadjuster 313.

In one embodiment, control circuit 333 generates a switching waveforms335, which is coupled to be received by power switch 147. In oneembodiment, power switch 147 is coupled between drain terminal 141 andsource terminal 143 to control a current flowing through the primarywinding 111 of power supply 101, which is coupled to drain terminal 141.

In one embodiment, negative current sensor 301 senses current that flowsout of negative current sensor 301 through multi-function terminal 149.Negative current sense signal 303 is generated in response to thecurrent that flows from negative current sensor 301 throughmulti-function terminal 149. In one embodiment, current that flows fromnegative current sensor 301 through multi-function terminal 149typically flows through an external resistance or switch coupled betweenmulti-function terminal 149 and ground.

In one embodiment, positive current sensor 305 senses current that flowsinto positive current sensor 305 through multi-function terminal 149.Positive current sense signal 307 is generated in response to thecurrent that flows into positive current sensor 305 throughmulti-function terminal 149. In one embodiment, current that flows intopositive current sensor 305 through multi-function terminal 149typically flows through an external resistance coupled betweenmulti-function terminal 149 and the DC line voltage 109 input to theprimary winding 111 of a power supply 101 and/or another voltage source.In another embodiment the current flows through an external resistanceor a switch coupled between the multi-function terminal 149 and anothervoltage source. In one embodiment, the line voltage 109 input to primarywinding 111 is typically a rectified and filtered AC mains signal.

As mentioned above, in one embodiment, positive current does not flowwhile negative current flows, and vice versa. In one embodiment, thenegative current sensor 301 and positive current sensor 305 are designedin such a way that they are not active at the same time . Stateddifferently, negative current sense signal 303 is not active at the sametime as positive current sense signal 307 in one embodiment.

In one embodiment, the voltage at multi-function terminal 149 is fixedat a first level when negative current flows out of power supplycontroller 139 from multi-function terminal 149. In one embodiment, thefirst level is selected to be approximately 1.25 volts. In oneembodiment, the voltage at multi-function terminal is fixed at a secondlevel when positive current flows into power supply controller 139through multi-function terminal 149. In one embodiment, the second levelis selected to be approximately 2.3 volts.

In one embodiment, on/off circuitry 309 generates on/off signal 311 inresponse to negative current sense signal 303. In one embodiment, whenthe current flowing from multi-function terminal 149 through an externalresistance to ground is less than a predetermined on/off thresholdlevel, on/off circuitry 309 generates on/off signal 311 to switch offthe power supply 101. In one embodiment, when the current flowing frommulti-function terminal 149 is greater than a predetermined on/offthreshold level, on/off circuitry 309 generates on/off signal 311 toswitch on the power supply 101. In one embodiment, the magnitude of theon/off threshold level is approximately 40 to 50 microamps, includinghysteresis.

In one embodiment, external current limit adjuster 313 generatesexternal current limit adjustment signal 315 in response to negativecurrent sense signal 303. In one embodiment, when the magnitude of thenegative current flowing from multi-function terminal 149 through anexternal resistance or switch to ground is below a predetermined level,the current limit adjuster 313 generates an external current limitadjustment signal to limit the current flowing through power switch 147.In one embodiment, when the magnitude of the negative current flowingfrom multi-function terminal 149 is below a predetermined level, thecurrent flowing through power switch 147 is limited to an amountdirectly proportional to the current flowing out of power supplycontroller 139 from multi-function terminal 149. In one embodiment,predetermined level is approximately 150 microamps. In one embodiment,if the magnitude of the negative current flowing out of power supplycontroller 139 from multi-function terminal 149 is greater than thepredetermined level, the current flowing through power switch 147 isinternally limited or clamped to a fixed safe maximum level. Therefore,the current flowing through power switch 147 is clamped to a safe value,even when multi-function terminal 149 is shorted to ground. In oneembodiment, the current flowing through power switch 147 is internallylimited or clamped to value of 3 amps.

In one embodiment, since the voltage at multi-function terminal 149 isfixed at a particular voltage when current flows out of power supplycontroller 139 through multi-function terminal 149, the current limitthrough power switch 147 can be accurately set externally with a singlelarge value, low-cost, resistor externally coupled betweenmulti-function terminal 149 and ground. By using a large externalresistance, the current flowing from multi-function terminal 149 isrelatively small. As mentioned above, the current flowing frommulti-function terminal 149 in one embodiment is in the microamp range.Since the current flowing from multi-function terminal 149 is relativelysmall, the amount of power dissipated is also relatively small.

In one embodiment, multi-function terminal 149 is coupled to the DC linevoltage 109 input to the primary winding 111 through an externalresistance. In one embodiment, the amount of current flowing intomulti-function terminal 149 represents the DC input line voltage to thepower supply 101. In one embodiment, under-voltage comparator 317generates under-voltage signal 319 in response to the resulting positivecurrent sense signal 307. In one embodiment, when the current flowinginto multi-function terminal 149 rises above a first predeterminedthreshold, under-voltage comparator 317 generates under-voltage signal319 to enable the power supply. In one embodiment, when the currentflowing into multi-function terminal 149 falls below a secondpredetermined threshold, under-voltage comparator 317 generatesunder-voltage signal 319 to disable the power supply. In one embodiment,the first predetermined threshold is greater than the secondpredetermined threshold to provide hysteresis. By providing hysteresisor a hysteretic threshold, unwanted switching on and off of the powersupply 101 resulting from noise or ripple is reduced. In one embodiment,the first predetermined threshold is approximately 50 microamps and thesecond predetermined threshold is approximately 0 microamps. In anotherembodiment, a hysteretic threshold is not utilized. Thus the hysteresisis greater than or equal to zero.

In one embodiment, over-voltage comparator 321 generates overvoltagesignal 323 in response to the positive current sense signal 307. In oneembodiment, when the current flowing into multi-function terminal 149rises above a third predetermined threshold, over-voltage comparator 321generates over-voltage signal 323 to disable the power supply. In oneembodiment, when the current flowing into multi-function terminal 149falls below a fourth predetermined threshold, over-voltage comparator321 generates over-voltage signal 323 to enable the power supply. In oneembodiment, the third predetermined threshold is greater than the fourthpredetermined threshold to provide hysteresis. By providing hysteresisor a hysteretic threshold, unwanted switching on and off of the powersupply resulting from noise is reduced. In one embodiment, the thirdpredetermined threshold is approximately 225 microamps and the fourthpredetermined threshold is approximately 215 microamps. In oneembodiment, the third and fourth predetermined thresholds are selectedto be approximately four to five times greater than the firstpredetermined threshold discussed above for an AC mains input 103 ofapproximately 85 volts to 265 volts AC. In another embodiment, ahysteretic threshold is not utilized. Thus the hysteresis is greaterthan or equal to zero.

In one embodiment, power switch 147 is able to tolerate higher voltageswhen not switching. When the power supply is disabled, power switch 147does not switch. Therefore, it is appreciated that over-voltagecomparator 321 helps to protect the power supply 101 from unwanted inputpower surges by disabling the power switch 147.

In one embodiment, over-voltage and under-voltage comparators 321 and317 may also be used for on/off functionality, similar to on/offcircuitry 309. In particular, multi-function terminal 149 may beswitchably coupled to a on/off control signal source to provide apositive current that flows into multi-function terminal 149 that crossthe under-voltage or over-voltage thresholds (going above the third orbelow the fourth predetermined thresholds). For example, when thepositive current through the multi-function pin crosses above the firstpredetermined threshold of the under-voltage comparator 317, the powersupply will be enabled and when the positive current goes below thesecond predetermined threshold of the under-voltage comparator 317, thepower supply is disabled. Similarly, when the positive current throughthe multi-function pin crosses above the third predetermined thresholdof the over-voltage comparator 321, the power supply will be disabledand when the positive current goes below the fourth predeterminedthreshold of the over-voltage comparator 321, the power supply isenabled.

In one embodiment, maximum duty cycle adjuster 325 generates maximumduty cycle adjustment signal 327 in response to the positive currentsense signal 307. In one embodiment, maximum duty cycle adjustmentsignal 327, which is received by control circuit 333, is used to adjustthe maximum duty cycle of the switching waveform 335 used to controlpower switch 147. In one embodiment, the maximum duty cycle determineshow long a power switch 147 can be on during each cycle. For example, ifthe maximum duty cycle is 50 percent, the power switch 147 can be on fora maximum of 50 percent of each cycle.

Referring briefly for example to the power supply 101 of FIG. 1, whilepower switch 147 is on, power is stored in the transformer core throughthe primary winding 111. While the power switch 147 is off, power isdelivered from the transformer core to the secondary winding of thetransformer in power supply 101. To delivery a given power level, for alower DC input voltage 109, a higher duty cycle is required and for ahigher DC input voltage 109, a lower duty cycle is required. In oneembodiment of the present invention, maximum duty cycle adjuster 325decreases the maximum duty cycle of power switch 147 in response toincreases in the DC input voltage 109. In one embodiment, maximum dutycycle adjuster 325 increases the maximum duty cycle of power switch 147in response to decreases in the DC input voltage 109. Stateddifferently, the maximum duty cycle is adjusted to be inverselyproportional to the current that flows into multi-function terminal 149in one embodiment of the present invention.

Referring back to FIG. 3, in one embodiment, the maximum duty cycle isadjusted within a range of 33 percent to 75 percent based on the amountof positive current that flows into multi-function terminal 149. In oneembodiment, maximum duty cycle adjuster 325 does not begin to decreasethe maximum duty cycle until the amount of current that flows intomulti-function terminal 149 rises above a threshold value. In oneembodiment, that threshold value is approximately 60 microamps. In oneembodiment, the maximum duty cycle is not adjusted if negative currentflows out of multi-function terminal 149. In this case, the maximum dutycycle is fixed at 75 percent in one embodiment of the present invention.

In one embodiment, enable/disable logic 329 receives as input on/offsignal 311, under-voltage signal 319 and over-voltage signal 323. In oneembodiment, if any one of the under-voltage or over-voltage conditionsexist, enable/disable logic 329 disables power supply 101. In oneembodiment, when the under-voltage and over-voltage conditions areremoved, enable/disable logic 329 enables power supply 101. In oneembodiment, power supply 101 may be enabled or disabled by starting andstopping, respectively, the switching waveform 335 at the beginning of aswitching cycle just before the power switch is to be turned on. In oneembodiment, enable/disable logic 329 generates enable/disable signal331, which is received by the oscillator in the control circuit 333 tostart or stop the oscillator at the beginning of a switching cycle ofswitching waveform 335. When enabled the oscillator will start a new oncycle of the switching waveform. When disabled the oscillator willcomplete the current switching cycle and stop just before the beginningof the next cycle.

In one embodiment, control circuit 333 generates switching waveform 335to control power switch 147 in response to a current sense signalreceived from drain terminal 141, enable/disable signal 331, maximumduty cycle adjustment signal 327, a control signal from control terminal145 and external current limit adjustment signal 315.

In one embodiment, the enable/disable signal can also be used tosynchronize the oscillator in the control circuit to an external on/offcontrol signal source having a frequency less than that of theoscillator. The on/off control signal can be input to the multi-functionterminal through any of the three paths that generate the enable/disablesignal: on/off circuitry 309, under-voltage comparator 317 orover-voltage comparator 321. As discussed, enable/disable the oscillatorin the control circuit 333, in one embodiment, begins a new completecycle of switching waveform at 335 using known techniques in response toenable/disable signal 331, which represents the on/off control signal atthe multi-function input. By turning the on/off control signal “on” atthe multi-function input for a fraction of the switching cycle and then“off,” the oscillator is enabled to start a new complete cycle.Therefore, if the external on/off control signal has short “on” pulsesat a frequency less than the oscillator in the control circuit, theoscillator will produce a switching cycle each time an on pulse isdetected, thus providing a switching waveform that is synchronized tothe external frequency.

In an alternate embodiment shown below in FIG. 7, the enable/disablesignal 331 directly disables or turns off the power switch through theAND gate 493 when an under-voltage or over-voltage condition exists. Inthis embodiment, the power switch can be enabled or disabled in themiddle of a cycle and consequently, synchronization of the switchingwaveform through a on/off control signal at the multi-function input isnot provided.

FIG. 4 is a schematic of one embodiment of a power supply controller 139in accordance with the teachings of the present invention. Asillustrated, negative current sensor 301 includes a current source 401coupled to control terminal 145. Transistors 403 and 405 form a currentmirror coupled to current source 401. In particular, transistor 403 hasa source coupled to current source 401 and a gate and drain coupled tothe gate of transistor 405. The source of transistor 405 is also coupledto current source 401. Transistor 407 is coupled between the drain andgate of transistor 403 and multi-function terminal 149. In oneembodiment, the gate of transistor 407 is coupled to a band gap voltageV_(BG) Plus a threshold voltage V_(TN). In one embodiment, V_(BG) isapproximately 1.25 volts, V_(TN) is approximately 1.05 volts andV_(BG)+V_(TN) is approximately 2.3 volts. Transistors 411 and 413 alsoform a current mirror coupled to the drain of transistor 405. Inparticular, the gate and drain of transistor 411 are coupled to thedrain of transistor 405 and the gate of transistor 413. In oneembodiment, negative current sense signal 303 is generated at the gateand drain of transistor 411. The sources of transistors 411 and 413 arecoupled to ground. In one embodiment, ground is provided through sourceterminal 143.

In one embodiment, on/off circuitry 309 includes a current source 409coupled between the drain of transistor 413 and control terminal 145. Inone embodiment, on/off signal 311 is generated at the drain oftransistor 413.

In one embodiment, external current limit adjuster 313 includes acurrent source 415 coupled between control terminal 145 and the drain oftransistor 419 and the gate and drain of transistor 421. The source oftransistor 419 and the source of transistor 421 are coupled to ground.The gate of transistor 419 is coupled to receive negative current sensesignal 303. External current limit adjuster 313 also includes a currentsource 417 coupled between control terminal 145 and the drain oftransistor 423 and resistor 425. The source of transistor 423 andresistor 425 are coupled to ground. External current limit adjustmentsignal 315 is generated at the drain of transistor 423.

In one embodiment, positive current sensor 305 includes transistor 429having a source coupled to multi-function terminal 149 and the currentmirror formed with transistors 431 and 433. In particular, transistor431 has a gate and drain coupled to the drain of transistor 429 and thegate of transistor 433. Current source 435 is coupled between ground andthe sources of transistors 431 and 433. The gate of transistor 429 iscoupled to band gap voltage V_(BG). The drain of transistor 433 iscoupled to the current mirror formed with transistors 427 and 437. Inparticular, the gate and drain of transistor 427 are coupled to the gateof transistor 437 and the drain of transistor 433. The sources oftransistors 427 and 437 are coupled to control terminal 145. Positivecurrent sense signal 307 is generated at the gate and drain oftransistor 427.

In one embodiment, under-voltage comparator 317 includes a currentsource 439 coupled between the drain of transistor 437 and ground.Under-voltage signal 319 is generated at the drain of transistor 437.

In one embodiment, over-voltage comparator 321 includes a current source443 coupled between the drain of transistor 441 and ground. Transistor441 has a source coupled to control terminal 145 and a gate coupled toreceive positive current sense signal 307. Over-voltage signal 323 isgenerated at the drain of transistor 441.

In one embodiment, enable/disable logic 329 includes NOR gate 445 havingan input coupled to receive under-voltage signal 319 and an invertedinput coupled to receive on/off signal 311. Enable/disable logic 329also includes NOR gate 447 having an input coupled to receiveover-voltage signal 323 and an input coupled to an output of NOR gate445. Enable/disable signal 331 is generated at the output of NOR gate447.

In one embodiment, maximum duty cycle adjuster 325 includes a transistor449 having a source coupled to control terminal 145 and a gate coupledto receive positive current sense signal 307. Maximum duty cycleadjuster 325 also includes a current source 453 coupled between thedrain of transistor 449 and ground. A diode 451 is coupled to the drainof transistor 449 to produce maximum duty cycle adjustment signal 327.

In one embodiment, power switch 147 includes a power metal oxidesemiconductor field effect transistor (MOSFET) 495 coupled between drainterminal 141 and source terminal 143. Power MOSFET 495 has a gatecoupled to receive a switching waveform 335 generated by pulse widthmodulator 333.

In one embodiment, control circuit 333 includes a resistor 455 coupledto the control terminal 145. A transistor 457 has a source coupled toresistor 455 and a negative input of a comparator 459. A positive inputof comparator 459 is coupled to a voltage V, which in one embodiment isapproximately 5.7 volts. An output of comparator 459 is coupled to thegate of transistor 457. The drain of transistor 457 is coupled to diode451 and resistor 479. The other end resistor 479 is coupled to ground. Afilter is coupled across resistor 479. The filter includes a resistor481 coupled to resistor 479 and capacitor 483 coupled to resistor 481and ground. Capacitor 483 is coupled to a positive input of comparator477.

In one embodiment, control circuit 333 is a pulse width modulator, whichhas an oscillator 467 with three oscillating waveform outputs 471, 473and 475. Oscillator 467 also includes an enable/disable input 469coupled to receive enable/disable signal 331. In one embodiment, controlcircuit 333 also includes a voltage divider including resistors 461 and463 coupled between drain terminal 141 and ground. A node betweenresistors 461 and 463 is coupled to a positive input of a comparator465. A negative input of comparator 465 is coupled to receive externalcurrent limit adjustment signal 315.

In one embodiment, oscillating waveform output 471 is coupled to a firstinput of AND gate 493. Oscillating waveform output 473 is coupled to aset input of latch 491. Oscillating waveform output 475 is coupled to anegative input of comparator 477. An output of comparator 465 is coupledto a first input of AND gate 487. A leading edge blanking delay circuit485 is coupled between the output of NAND gate 493 and a second input ofAND gate 487. In one embodiment, there is a gate driver or a bufferbetween the output of the NAND gate 493 and the gate of the MOSFET (notshown). An output of AND gate 487 is coupled to a first input of OR gate489. A second input of OR gate 489 is coupled to an output of comparator477. An output of OR gate 489 is coupled to a reset input of latch 491.An output of latch 491 is coupled to a second input of AND gate 493. Theoutput of AND gate 493 generates switching waveform 335.

Operation of power supply controller 139 of FIG. 4 is as follows.Beginning with negative current sensor 301, the gate of transistor 407is fixed at V_(BG)+V_(TN) in one embodiment to approximately 2.3 volts.As a result, transistor 407 sets the voltage at multi-function terminal149 to V_(BG) in one embodiment, which is approximately 1.25 volts, whencurrent is pulled out of multi-function terminal 149. This current maybe referred to as negative current since the current is being pulled outof power supply controller 139. In one embodiment, transistor 407 issized such that it operates with a current density resulting in avoltage drop between the gate and source that is close to V_(TN),wherein the V_(TN) is the threshold of the N channel transistor 407,when negative current flows from multi-function terminal 149. When anexternal resistor (not shown) is coupled from multi-function terminal149 to ground, the negative current flowing through the externalresistor will therefore be V_(BG) divided by the value of the externalresistor in accordance with Ohm's law. This negative current flowing outfrom multi-function terminal 149 passes through transistors 403 and ismirrored on to transistor 405. Current source 401 limits the negativecurrent sourced by multi-function terminal 149. Therefore, even ifmulti-function terminal 149 is short-circuited to ground, the current islimited to a current less than the current supplied by current source401. This current is less than the current source 401 by an amount thatflows through the transistor 405. In one embodiment, the negativecurrent that can be drawn from the multi-function terminal is limited to200 microamps by the current source 401. In one embodiment, if morenegative current than current source 401 is able to supply is pulledfrom multi-function terminal 149, the voltage at multi-function terminal149 collapses to approximately 0 volts.

The current that flows through transistor 403 is mirrored to transistor405. The current that flows through transistors 405 and 411 is the samesince they are coupled in series. Since transistors 411 and 413 form acurrent mirror, the current flowing through transistor 413 isproportional to the negative current flowing through multi-functionterminal 149. The current flowing through transistor 413 is compared tothe current provided by current source 409. If the current throughtransistor 413 is greater than the current supplied by current source409, the signal at the drain of transistor 413 will go low, which in oneembodiment enables the power supply. Indeed, on/off signal 311 isreceived at an inverted input of NOR gate 445. Thus, when on/off signal311 is low, the power supply is enabled. Therefore, by having a negativecurrent greater than a particular threshold value, the power supply ofthe present invention is enabled in one embodiment. In one embodiment,the magnitude of that particular threshold value is approximately 50microamps.

As mentioned above, the current flowing through transistor 411 isproportional to the negative current flowing out from multi-functionterminal 149. As illustrated, transistor 419 also forms a current mirrorwith transistor 411. Therefore, the current flowing through transistor419 is proportional to the current flowing through transistor 411. Thecurrent flowing through transistor 421 is the difference between thecurrent supplied by current source 415 and the current flowing throughtransistor 419. For example, assume that the current supplied by currentsource 415 is equal to A. Assume further that the current flowingthrough transistor 419 is equal to B. In this case, the current flowingthrough transistor 421 is equal to A-B.

As illustrated, transistor 423 forms a current mirror with transistor421. Therefore, the current flowing through transistor 423 isproportional to the current flowing through transistor 421. Continuingwith the example above and assuming further that transistors 421 and 423are equal in size, the current flowing through transistor 423 is alsoequal to A-B. Assuming further that current source 417 supplies acurrent equal to the current supplied by current source 415, which isassumed to be equal to A, then the current flowing through resistor 425would be equal to A-(A-B), which is equal to B.

Therefore, the current flowing through resistor 425 is proportional tothe current flowing through transistor 419, which is proportional to thecurrent flowing through transistor 411, which is proportional to thenegative current flowing out from multi-function terminal 149. Note thatif the current flowing through transistor 419 is greater than thecurrent supplied by current source 415, the current flowing throughtransistor 421 would be zero because the voltage at the drains oftransistors 419 and 421 would collapse to approximately zero volts. Thiswould result in the current flowing through transistor 423 to be zero .Thus, the current through resistor 425 cannot be greater than thecurrent supplied by current source 417. However, as long as B is lessthan A, the current that flows through resistor 425 is equal to B. If Brises above A, the current that flows through resistor 425 is equal toA.

In one embodiment, resistor 425 is fabricated using the same or similartypes of processes and diffusions or doped regions used in fabricatingpower MOSFET 495. As a result, the on resistance of resistor 425 followsor tracks the on resistance of power MOSFET 495 through varyingoperating conditions and processing variations.

The voltage across resistor 425 is reflected in external current limitadjuster signal 315, which is input to the negative input of comparator465. In one embodiment, the negative input of comparator 465 is thethreshold input of comparator 465. Therefore, the negative input ofcomparator 465 receives a voltage proportional to the negative currentflowing out of multi-function terminal 149 multiplied by the resistanceof resistor 425.

The positive input of comparator 465 is coupled to drain terminal 141through resistor 461 of the voltage divider formed by resistor 461 andresistor 463. Therefore, the positive input of comparator 465 senses avoltage proportional to the drain current of power MOSFET 495 multipliedby the on resistance of power MOSFET 495.

When the voltage at the positive terminal of comparator 465 rises abovethe voltage provided by external current limit adjuster signal 315 tothe negative terminal of comparator 465, the output of comparator 465 isconfigured to reset latch 491 through AND gate 487 and OR gate 489. Byresetting latch 491, the on portion of a cycle of waveform 335 receivedat the gate of power MOSFET 495 is masked or cut short, which results inturning off power MOSFET 495 when the amount of current flowing throughpower switch 147 rises above the threshold.

In one embodiment, AND gate 487 also receives input from leading edgeblanking delay circuitry 485. In one embodiment, leading edge blankingdelay circuitry 485, using known techniques, temporarily disablescurrent limit detection at the start, or during the leading edgeportion, of an on transition of power MOSFET 495.

As shown in the embodiment illustrated in FIG. 4, latch 491 is set atthe beginning of each cycle by switching waveform output 473. One waythat latch 491 is reset, thereby turning off power MOSFET 495, isthrough the output of comparator 465. Another way to reset latch 491 isthrough the output of comparator 477, which will be discussed below inconnection with maximum duty cycle adjuster 325.

With regard to positive current sensor 305, the gate of transistor 429is coupled to the band gap voltage V_(BG). In one embodiment, transistor429 is sized such that it operates with a current density resulting in adrop between the source and gate close to V_(TP), which is threshold ofthe P channel transistor 429, when positive current flows intomulti-function terminal 149. In one embodiment, current that flows intomulti-function terminal 149 is referred to as positive current since thecurrent is being fed into the power supply controller 139. As a result,the voltage at multi-function terminal 149 is fixed at approximatelyV_(BG)+V_(TP) when positive current flows into multi-function terminal149.

The gate voltages on the transistors 407 and 429 chosen in theembodiment discussed above are such that only one of transistors 407 and429 are switched on at a time depending on the polarity of the currentat the multi-function terminal. Stated differently, if transistor 407 ison, transistor 429 is off. If transistor 429 is on, transistor 407 isoff. As result, if negative current sensor 301 is on, positive currentsensor 305 is isolated from multi-function terminal 149. If positivecurrent sensor 305 is on, negative current sensor 301 is isolated frommulti-function terminal 149. Therefore, if there is negative currentflowing through multi-function terminal 149, positive current sensor 305is disabled. If there is positive current flowing through multi-functionterminal 149, negative current sensor 301 is disabled.

In one embodiment, the positive current that flows into transistor 429flows through transistor 431 since they are coupled in series. Thepositive current through multi-function terminal 149 flows into and islimited by current source 435. In one embodiment, if the positivecurrent through multi-function terminal 149 is greater than an amountthat current source 435 can sink minus the current in transistor 433,then the voltage at multi-function terminal 149 will rise and is clampedeither by the circuitry driving the current or by the standard clampingcircuitry that is used for protection purposes on external terminalssuch as the multi-function terminal, of a power supply controller. Asshown, transistors 431 and 433 form a current mirror. Therefore, thecurrent flowing through transistor 433 is proportional to the positivecurrent that flows through transistor 431. The current that flowsthrough the transistor 433 flows to transistor 427 since they arecoupled in series. As shown, the gate of transistor 427 is coupled tothe drain of transistor 427, which generates positive current sensesignal 307.

Transistors 427 and 437 form a current mirror since the gate and drainof transistor 427 are coupled to the gate of transistor 437. Therefore,the current flowing through transistor 437 is proportional to thecurrent flowing through transistor 427, which is proportional to thepositive current. Current source 439 provides a reference current, whichis compared to the current that flows through transistor 437. If thecurrent flowing through transistor 437 rises above the current providedby current source 439, then the voltage at the drain of transistor 437,which is the under-voltage signal 319, goes high. When under-voltagesignal 319 goes high and the output of NOR gate 445 will go low,indicating that there is no under-voltage condition.

Transistors 427 and 441 also form a current mirror since the gate anddrain of transistor 427 are coupled to the gate of transistor 441.Therefore, the current flowing through transistor 441 is proportional tothe current flowing through the transistor 427, which is proportional tothe positive current. Current source 443 provides a reference current,which is compared to current that flows through transistor 441. As longas the current flowing through transistor 441 stays below the currentprovided by current source 443, then the voltage at the drain oftransistor 441, which is the over-voltage signal 323, remains low. Whenover-voltage signal 323 remains low, the output of NOR gate 447 remainshigh assuming that there was no under-voltage condition indicated byunder-voltage signal 319 and no remote off condition indicated by on/offsignal 311.

The output of NOR gate 447 is enable/disable signal 331. In oneembodiment, enable/disable signal 331 is high if on/off signal 311 islow, or under-voltage signal 319 is high and over-voltage signal 323 islow. Otherwise, enable/disable signal 331 is low.

In one embodiment, the oscillator 467 receives enable/disable signal 331at the start/stop input 469. In one embodiment, oscillator 467 generatesoscillating waveforms at oscillating waveform outputs 471, 473 and 475while enable/disable signal 331 is high or active. In one embodiment,oscillator 467 does not generate the oscillating waveforms atoscillating waveform outputs 471, 473 and 475 while enable/disablesignal 331 is low or in-active. In one embodiment, oscillator 467 beginsgenerating oscillating waveforms starting with new complete cycles on arising edge of enable/disable signal 331. In one embodiment, oscillator467 completes existing cycles of the oscillating waveforms generated atoscillating waveform outputs 471, 473 and 475 before stopping thewaveforms in response to a falling edge of enable/disable signal 331.That is, oscillator 467 stops generating the waveforms at a point justbefore the start of an on time of power switch of the next cycle inresponse to a falling edge of enable/disable signal 331.

In one embodiment, control terminal 145 supplies power to the circuitryof power supply controller 139 and also provides feedback to modulatethe duty cycle of switching waveform 335. In one embodiment, controlterminal 145 is coupled to the output of the power supply 101 through afeedback circuit to regulate the output voltage of the power supply 101.In one embodiment, an increase in the output voltage of power supply 101results in the reduction in the duty cycle of switching waveform 335through feedback received through control terminal 145. Therefore, ifthe regulation level of the output parameter of power supply 101 that isbeing controlled, such as output voltage or current or power, isexceeded during operation, additional feedback current is receivedthrough control terminal 145. This feedback current flows throughresistor 455 and through a shunt regulator formed by transistor 457 andcomparator 459. If no feedback current or control terminal current inexcess of supply current is received through control terminal 145, thecurrent through transistor 457 is zero. If the current throughtransistor 457 is zero, and assuming for the time being that there is nocurrent through the diode 451, the current through resistor 479 is zero.If there is no current flowing through resistor 479, then the voltagedrop across resistor 479 is zero. If there is no voltage drop acrossresistor 479, there is no voltage drop across capacitor 483. As aresult, the output of comparator 477 will remain low. If the output ofcomparator 477 remains low, and assuming for the time being that theoutput of AND gate 487 remains low, the output of latch 491 will remainhigh. In this case, the maximum duty cycle signal, which is produced byoscillator waveform output 471, will be generated at the output of ANDgate 493. Thus, switching waveform 335 will have the maximum duty cycleproduced by oscillator waveform output 471.

Therefore, when the voltage drop across resistor 479 remains at zero,the maximum duty cycle produced at oscillator waveform output 471 is notlimited, assuming that the output of AND gate 487 remains low. This isbecause latch 491 is not reset through the output of comparator 477.However, when the feedback current or control terminal current in excessto the supply current is received through control terminal 145, thisfeedback current flows through transistor 457. As the amount of currentflowing through the transistor 457 increases, the voltage drop acrossresistor 479 increases correspondingly. As a voltage drop acrossresistor 479 increases, the voltage drop across capacitor 483 willincrease. In any given cycle, when the voltage on the oscillatingwaveform output 475 crosses below the voltage across the capacitor 483the output of the comparator will go high and terminate the on-time ofthe switching waveform 335 or turn off the power switch 495. As aresult, the duty cycle (on time as a fraction of the cycle time) of theswitching waveform 335 decreases with increase in voltage drop acrossresistor 479.

In one embodiment, the oscillating waveform at oscillating waveformoutput 475 is a sawtooth waveform having a duty cycle and period equalto the maximum duty cycle waveform generated at oscillating waveformoutput 471. As the voltage drop across resistor 479 increases, theoutput of comparator 477 will go high closer to the beginning of eachcycle. When the output of comparator 477 goes high, latch 491 will bereset through NOR gate 489. When latch 491 is reset, the on time of theof switching waveform 335 is terminated for that particular cycle andswitching waveform 335 remains low for the remainder of that particularcycle. Latch 491 will not be set again until the beginning of the nextcycle through switching waveform output 473, assuming that there is ahigh or active enable/disable signal 331.

Referring now to maximum duty cycle adjuster signal 325, transistor 449includes a source coupled to control terminal 145 and a gate coupled thegate and drain of transistor 427 to receive positive current sensesignal 307. Transistor 449 and transistor 427 also form a currentmirror. Therefore, the current flowing through transistor 449 isproportional to the current flowing through transistor 427, which isproportional to the positive current flowing into multi-functionterminal 149. The current that flows through diode 451 is the differencebetween the current that flows through transistor 449 and the currentthat flows into current source 453. The current that flows throughcurrent source 453 is set such that current will not begin to flowthrough diode 451 until the current flowing through transistor 449 risesabove a threshold. In one embodiment, the above threshold value ischosen such that the maximum duty cycle is not reduced until thepositive current flowing into multi-function terminal 149 rises abovethe threshold used for under-voltage comparison. In one embodiment, thethreshold positive current used for under-voltage comparison isapproximately 50 microamps and the threshold positive current used formaximum duty cycle adjustment is approximately 60 microamps.

When current begins to flows through diode 451, that current will becombined with current that flows through transistor 457. In oneembodiment, the current that flows through diode 451 is maximum dutycycle adjustment signal 327. The current flowing through transistor 457and diode 451 will flow through resistor 479. As discussed in detailabove, current that flows through resistor 479 will result in thevoltage drop across resistor 479, which results in a reduction in themaximum duty cycle of switching waveform 335. As the current that flowsthrough resistor 479 increases, the maximum duty cycle of switchingwaveform 335 will be decreased.

FIG. 5 is a diagram illustrating some of the currents, voltages and dutycycles associated with the power supply controller 139 in accordancewith teachings of the present invention. In particular, diagram 501illustrates when the power supply is enabled in relation to the inputcurrent of multi-function terminal 149. The x-axis represents thepositive or negative current flowing into or out of multi-functionterminal 149. As illustrated, as positive input current rises from zeroand crosses over 50 microamps, power supply controller 139 in oneembodiment is enabled. At this time, an under-voltage condition isremoved. If the current is above 50 microamps but then falls below zeromicroamps, power supply controller 139 is disabled. At this time, anunder-voltage condition is detected. The difference between 50 microampsand zero microamps provides hysteresis, which provides for more stableoperation during noise or ripple conditions in the input current.

As the input current rises above 225 microamps, the power supply isdisabled. At this time, an over-voltage condition is detected. When theinput current falls back below 215 microamps, the power supply isre-enabled. At this time, the over-voltage condition is removed. Thedifference between 225 microamps and 215 microamps provides hysteresis,which provides for more stable operation during noise or rippleconditions in the input current.

Continuing with diagram 501, when the negative current that flows outfrom multi-function terminal 149 rises in magnitude to a level above 50microamps, which is illustrated as −50 microamps in FIG. 5, the powersupply is enabled. At this time, the on/off feature of the presentinvention turns on the power supply. When the negative current falls inmagnitude to a level below 40 microamps, which is illustrated as −40microamps in FIG. 5, the power supply is disabled. At this time, theon/off feature of the present invention turns off the power supply. Thedifference between −50 microamps and −40 microamps provides hysteresis,which provides for more stable operation during noise or rippleconditions in the input current.

It is worthwhile to note that in one embodiment the positive inputcurrent is clamped at 300 microamps and that the negative input currentis clamped at 200 microamps. The positive input current would be clampedat 300 microamps when, for example, the multi-function terminal 149 isshort-circuited to a supply voltage. The negative input current would beclamped at 200 microamps when, for example, the multi-function terminalis short-circuited to ground.

In diagram 503, the current limit through power switch 147 as adjustedby the present invention is illustrated. Note that the hysteresis of theunder-voltage and over-voltage conditions are illustrated from zeromicroamps to 50 microamps and from 215 microamps to 225 microamps,respectively. In one embodiment, when positive input current is providedinto multi-function terminal 149 and there is neither an under-voltagecondition nor an over-voltage condition, the current limit through powerswitch 147 is 3 amps. However, when negative current flows out frommulti-function terminal 149, and the magnitude of the negative currentrises above 50 microamps, which is illustrated as −50 microamps in FIG.5, the current limit through power switch 149 is approximately 1 amp. Asthe magnitude of the negative current rises to 150 microamps, which isillustrated as −150 microamps in FIG. 5, the current limit through powerswitch 149 rises proportionally with the negative current to 3 amps.After the magnitude of the negative current rises above 150 microamps,the current limit of the power switch 149 remains fixed at 3 amps. Notethat there is also the on/off hysteresis between −50 microamps and −40microamps in diagram 503.

Diagram 505 illustrates the maximum duty cycle setting of power supplycontroller 139 in relation to the input current. Note that thehysteresis from −50 microamps to −40 microamps, from zero microamps to50 microamps and from to 215 microamps to 225 microamps as discussedabove is included. In the embodiment illustrated in diagram 505, themaximum duty cycle is fixed at 75 percent until a positive input currentof 60 microamps is reached. As the input current continues to increase,the maximum duty cycle continues to decrease until an input current of225 microamps is reached, at which time the maximum duty cycle has beenreduced to 33 percent. As illustrated, between 60 microamps and 225microamps, the maximum duty cycle is inversely proportional to thepositive input current. Note that when negative current flows throughmulti-function terminal 149, and when the power supply is enabled, themaximum duty cycle in one embodiment is fixed at 75 percent.

Diagram 507 illustrates the voltage at multi-function terminal, which islabeled in diagram 507 as line sense voltage, in relation to the inputcurrent. When negative current is flowing from multi-function terminal149, the voltage at multi-function terminal 149 is fixed at the band gapvoltage V_(BG), which in one embodiment is 1.25 volts. When positivecurrent is flowing into multi-function terminal 149, the voltage atmulti-function terminal is fixed at the band gap voltage V_(BG) plus athreshold voltage V_(TP), which in one embodiment sum to 2.3 volts. Inthe event that a negative current having a magnitude of more than 200microamps is attempted to be drawn out of the multi-function terminal149, the voltage at multi-function terminal 149 drops to approximatelyzero volts. In the event that a positive current of more than 300microamps flows into multi-function terminal 149, the voltage atmulti-function terminal 149 rises. In this case, the voltage will belimited by either by a standard clamp used at the multi-functionterminal for the purpose of protection or by the external circuitrydriving the multi-function terminal, whichever is lower in voltage.

It is appreciated that the currents, voltages, duty cycle settings andhysteresis settings described in connection with the present inventionare given for explanation purposes only and that other values may beselected in accordance with teachings of the present invention. Forexample, in other embodiments, non hysteretic thresholds may beutilized. Thus the hysteresis values may be greater than or equal tozero.

FIG. 6A is timing diagram illustrating one embodiment of some of thewaveforms of a power supply controller in accordance with teachings ofthe present invention. Referring to both FIGS. 4 and 6A, oscillatingwaveform output 475 of oscillator 467 generates a sawtooth waveform,which is received by comparator 477. Oscillating waveform output 471 ofoscillator 467 generates a maximum duty cycle signal, which is receivedby AND gate 493. Enable/disable signal 331, which is received atenable/disable input 469 of oscillator 467, is also illustrated. In FIG.6A, the enable/disable signal 331 is active. Therefore, the sawtoothwaveform of oscillating waveform output 475 and the maximum duty cyclewaveform of oscillating waveform output 471 are generated. Note that thesawtooth waveform and the maximum duty cycle waveform have the samefrequency and period. One cycle of each of these waveform occurs betweentime 601 and time 605. The peak of the sawtooth waveform occurs at thesame time as the rising edge of the maximum duty cycle waveform. Thisaspect is illustrated at time 601 and at time 605. The lowest point ofthe sawtooth waveform occurs at the same time as the falling edge of themaximum duty cycle waveform. This aspect is illustrated at time 603.

Referring now to FIG. 6B, a timing diagram illustrating anotherembodiment of the waveforms of a power supply controller in accordancewith teachings of the present invention is shown. From time 607 to time609, the enable/disable signal 331 is low or inactive. In oneembodiment, a low enable/disable signal 331 disables the power supply. Ahigh or active enable/disable signal 331 enables the power supply. Attime 609, the rising edge of enable/disable signal 331 occurs. At thistime, oscillating waveform outputs 475 and 471 begin generating thesawtooth waveform and maximum duty cycle waveform, respectively. Notethat a new complete cycle of each of these waveforms is generated inresponse to the rising edge of enable/disable signal 331 at time 609.

From time 609 to time 611, enable/disable signal 331 remains high oractive. Thus, during this time, the sawtooth waveform and maximum dutycycle waveform are continuously generated.

At time 611, a falling edge of enable/disable signal 331 occurs. Beforeoscillator 467 discontinues generating the sawtooth waveform and themaximum duty cycle waveform, the existing cycles of each of thesewaveforms are allowed to complete. Stated differently, generation of thesawtooth waveform and the maximum duty cycle waveform is discontinued ata point just before the start of the on-time of the switching waveform335 or the on-time of the power switch of the next cycle. This point intime is illustrated in FIG. 6B at time 613. Note that after time 613,the sawtooth waveform remains inactive at a high value and the maximumduty cycle waveform remains inactive at a low value.

At time 615, another rising edge of enable/disable signal 331 occurs.Therefore, the sawtooth waveform and the maximum duty cycle waveform aregenerated beginning at a new complete cycle of each waveform. Asillustrated in FIG. 6B, a falling edge of enable/disable signal 331occurs at time 617, which is immediately after the rising edge. However,the sawtooth waveform and maximum duty cycle waveforms are allowed tocomplete their then existing cycles. This occurs at time 619. After time619, the waveforms remains inactive as shown during the time betweentime 619 and time 621, which is when another rising edge ofenable/disable signal 331 occurs. At time 621, another new completecycle of the sawtooth waveform and the maximum duty cycle waveform aregenerated. Since enable/disable signal 331 is deactivated at time 623,which occurs during a cycle of the sawtooth waveform and the maximumduty cycle waveform, these waveforms are deactivated after fullycompleting their respective cycles. Thus, by pulsing the on/off controlsignal at the multi-function terminal it is possible to synchronize theoscillator to the on/off pulse frequency.

FIG. 7 is a schematic of another embodiment of a power supply controller139 in accordance with the teachings of the present invention. The powersupply controller schematic shown in FIG. 7 is similar to the powersupply controller schematic discussed above in FIG. 4. The primarydifference between the power supply controller of FIGS. 4 and 7 is thatoscillator 467 of FIG. 7 does not have an enable/disable input 469coupled to receive enable/disable signal 331. As shown in the embodimentdepicted in FIG. 7, the enable/disable signal 331 is used to directlygate the switching waveform at the input of AND gate 493. In thisembodiment, the oscillator 467 is running all the time and switchingwaveform 335 will be gated on and off at any point in the cycle inresponse to the enable/disable signal 331.

To illustrate, FIG. 8 shows one embodiment of timing diagrams ofswitching waveforms of the power supply controller illustrated in FIG.7. Referring to both FIGS. 7 and 8, oscillating waveform output 475 ofoscillator 467 generates a sawtooth waveform, which is received bycomparator 477. Oscillating waveform output 471 of oscillator 467generates a maximum duty cycle signal, which is received by AND gate493. Enable/disable signal 331, which is received by AND gate 493, andthe output of AND gate 493, which is switching waveform 335, are alsoillustrated. In FIG. 8, the enable/disable signal 331 is active onlysome of the time. Therefore, the switching waveform 335 is switchingonly during those portions of time that the enable/disable signal 331 isactive. When the enable/disable signal 331 is not active, switchingwaveform 335 does not switch.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A power supply controller circuit, comprising: acurrent input circuit coupled to receive a current for adjusting acurrent limit of a power switch, the current input circuit to generate acurrent limit adjustment signal in response to the current; and acontrol circuit coupled to receive the current limit adjustment signal,the control circuit coupled to adjust the current limit of a currentthrough the power switch in response to the current limit adjustmentsignal.
 2. The power supply controller circuit of claim 1 wherein thepower switch is coupled to a primary winding of the power supply.
 3. Thepower supply controller circuit of claim 1 wherein the control circuitis a pulse width modulation circuit that generates a switching waveformcoupled to be received by the power switch to regulate a power supplyoutput.
 4. The power supply controller of claim 3 wherein the current isrepresentative of a feedback signal from the power supply output,wherein a power supply voltage is regulated through current limitadjustment of the power switch in response to the feedback signal. 5.The power supply controller of claim 3 wherein the control circuitincludes a first comparator coupled to compare a voltage representativeof the current through the power switch with the current limitadjustment signal such that the power switch is disabled in response toan output of the first comparator when the current limit set by thecurrent limit adjustment signal is exceeded.
 6. The power supplycontroller of claim 5 wherein the control circuit is to generate saidswitching waveform controlled in response to the output of the firstcomparator such that the switching waveform is coupled to limit thecurrent through the power switch.
 7. The power supply controller circuitof claim 1 wherein the current limit of the current through the powerswitch is adjusted by the current when the current limit of the currentthrough the power switch is below a predetermined maximum level.
 8. Thepower supply controller circuit of claim 7 wherein the current limit ofthe current through the power switch is fixed at the predeterminedmaximum level for magnitudes of the currents that are higher than thecurrent value corresponding to the predetermined maximum level.
 9. Thepower supply controller of claim 1, wherein the current circuit alsogenerates an enable/disable signal that deactivates the power supplywhen the magnitude of the current is below an on/off threshold, theon/off threshold having a hysteresis of zero or greater.
 10. The powersupply controller circuit of claim 9 further comprising an oscillatorcircuit coupled to an enable/disable signal, the oscillator circuit tostart and stop generating a switching waveform in response to thecurrent crossing the on/off threshold.
 11. The power supply controllercircuit of claim 10 wherein the oscillator circuit is to complete anexisting cycle of the switching waveform before the oscillator is tostop generating the switching waveform in response to the enable/disablesignal.
 12. The power supply controller circuit of claim 10 wherein theoscillator circuit is to start a new complete cycle of the switchingwaveform if the oscillator circuit is to start generating the switchingwaveform in response to the enable/disable signal.
 13. The power supplycontroller circuit of claim 1, wherein the current is received by thecurrent input circuit on a low impedance terminal that has a referencevoltage with respect to ground.
 14. The power supply controller in claim13, wherein the current limit of the power switch is set by the value ofresistance connected between the reference voltage on the low impedanceterminal and ground.
 15. A method for controlling a power supply,comprising: supplying a first current from a first terminal of a powersupply controller; deactivating the power supply if the first currentsupplied from the first terminal falls below a first threshold value;and activating the power supply if the first current supplied from thefirst terminal rises above a second threshold value.
 16. The method ofclaim 15 wherein deactivating the power supply comprises stopping aswitching waveform to control a power switch coupled to a primarywinding of the power supply.
 17. The method of claim 15 whereinactivating the power supply comprises starting a switching waveform tocontrol a power switch coupled to a primary winding of the power supply.18. The method of claim 17 wherein starting the switching waveformincludes starting a new complete cycle of the switching waveform. 19.The method of claim 15 wherein the second threshold value is greaterthan the first threshold value.
 20. The method of claim 15 furthercomprising limiting the first current supplied from the first terminalto a maximum value.
 21. The method of claim 16 wherein stopping theswitching waveform includes allowing to complete an existing cycle ofthe switching waveform.
 22. The method of claim 15 further comprisingcoupling a switch between the first terminal and ground.
 23. The methodof claim 15 further comprising coupling a variable resistance betweenthe first terminal and ground.
 24. A method for controlling a powersupply, comprising: supplying a first current from a first terminal of apower supply controller; controlling a second current flowing through aprimary winding of the power supply with a power switch coupled to theprimary winding; and adjusting a current limit of the second current inresponse to the first current.
 25. The method of claim 24 whereinadjusting the current limit of the second current comprises increasingthe current limit of the second current in response to an increase inthe first current.
 26. The method of claim 24, wherein adjusting thecurrent limit of the second current comprises decreasing the currentlimit of the second current in response to a decrease in the firstcurrent.
 27. The method of claim 24 further comprising coupling aresistance between the first terminal and ground.
 28. The method ofclaim 24 wherein controlling a second current flowing through theprimary winding comprises: switching the power switch in response to aswitching waveform; and adjusting the switching waveform in response tothe first current.
 29. The method of claim 28 wherein adjusting theswitching waveform comprises: generating a first voltage in response tothe first current; generating a second voltage in response to the secondcurrent; and adjusting the switching waveform in response to acomparison of the first voltage and the second voltage.